1. Field of the Invention
The present invention generally relates to a field-sequential color display unit and display method.
2. Description of Related Art
Conventionally, typical color displays are designed to carry out a color display on the basis of spatially additive color mixing system. In general, the spatially additive color mixing system is a method for arranging three-primary colors, i.e., red (which will be hereinafter referred to as R), green (which will be hereinafter referred to as G) and blue (which will be hereinafter referred to as B), in parallel so that the observer can not recognize such a state that R, G and B are spatially divided, and the primary colors are varied the ratio of each intensity and mixed in the observer's eyes in order to display color images. In contrast to this method, in recent years, displays based on the field-sequentially additive color mixing system are being actively developed. In the case of the spatially additive color mixing system, it is required to divide one pixel into three sub-pixels corresponding to red, green and blue (RGB) pixel in order to carry out color displaying. On the other hand, in the case of the field-sequentially additive color mixing system, color displaying can be carried out with one pixel. Therefore, the field-sequentially additive color mixing system is widely noticed as one of the methods for increasing there solution of displays. In contrast to the spatially additive color mixing system, the field-sequentially additive color mixing system is designed to temporally divide with every input picture into the three-primary color's displaying periods, and display the divided periods sequentially at such a speed that the observer can not recognize the divided periods, to carry out color displaying. A display unit utilized the field-sequentially additive color mixing system is generally called a field-sequential color display unit.
There are field-sequential color display units having various systems, such as a color shutter system or a backlight system illuminating the three-primary colors. In all systems, the field-sequential color display unit is designed to divide a set of signals of each input picture into R, G and B signals, which are signals indicative of the three-primary colors, in order to sequentially display R, G and B images during one frame period at the triple speed to carry out color displaying. That is, in the field-sequential color display unit, one frame period, which is a time required to complete the update each color image displaying, comprises a plurality of fields which display each color information. Each of the field periods will be hereinafter referred to as a sub-field in order to distinguish it from a field period in an interlacing display. When the interlacing display is carried out using the field-sequential color display unit, one field generally comprises three sub-fields of the primary colors R, G and B, and one frame comprises even-odd two fields. In order to simplify discussion, if there is no particular explanation, it is hereinafter assumed that the non-interlacing display is a premise, which means one frame equals to one field, and one frame comprises a plurality of sub-field.
In a typical display unit, one frame frequency must be displayed at the critical fusion frequency (CFF) or at a higher frequency so as a flicker cannot be recognized. Therefore, in the field-sequential color display unit, each sub-field must be displayed at the frequency of N times as many as a frame frequency wherein the number of sub-fields per frame is N. For example, as shown in FIG. 25, assuming that one frame frequency is 60 Hz, a field-sequential color display requires three sub-fields for RGB per frame; each sub-field frequency is 180 Hz.
In order to realize the field-sequential color display, there is used means for temporally filtering a monochrome image by an RGB filter or means for temporally switching illumination of a plurality of RGB light sources. Specifically, as examples of the former, there are constructions wherein a white light source illuminates a light valve and an RGB disk color filter (color wheel) is mechanically rotated and wherein monochrome (black and white) images are displayed on a monochrome CRT (Cathode Ray Tube) and a liquid crystal color shutter is provided in front of the CRT. As an example of the latter, there is provided a construction wherein a light valve is illuminated with RGB-colorized illumination by LEDs (Light Emitting Diodes) or a set of cold cathode fluorescent lamps.
From the aforementioned reasons, the field-sequential color display requires a higher refresh rate than the display based on the spatially additive color mixing system. Therefore, it is desirable that the light valve for displaying images uses a display device having fully rapid response time, such as a DMD (Digital Micro-mirror Device), a bend alignment liquid crystal cell (including a PI twisted cell, and OCB (Optically Compensated Birefringence) mode in which a phase compensating film is added), a FLC (Ferroelectric Liquid Crystal) cell using liquid crystal materials in the smectic phase including SSFLC (Surface Stabilized Ferroelectric Liquid Crystal) cell, an AFLC (Antiferroelectric Liquid Crystal) cell including a V-shaped response liquid crystal cell (which is frequently called TLAF (ThresholdLess Anti-Ferroelectric) mode wherein a voltage-transmittance curve indicates a thresholdless V-shaped response). Generally, most of the liquid cell modes used for the liquid crystal color shutter is able to use for the display device.
Therefore, in the field-sequential color display, the lower limit of the sub-field frequency at which a flicker cannot be perceived is 3 times of the CFF, i.e., about 150 Hz. It is known that the “color breakup artifact” occurs if the sub-field frequency is lower than the limit. This is interference that the profile of an image or screen is seen so as to be colorized since the RGB-images are time-integrated without being coincident with each other on a retina due to the eye movement following a moving picture, blink or saccade of an eye.
For example, if the frame frequency 60 Hz, each of RGB-sub-fields is displayed at 180 Hz. If the observer watches a static image, the RGB-colors of the sub-field images are mixed on the observer's retinas at 180 Hz, so that a true color image can be presented to the observer. When a white box 210 is displayed on the screen as shown in FIG. 26(a), the colors of the sub-field images of red, green and blue are mixed on the observer's retinas and presented a true color image to the observer. However, when the observer's eyes move across the display screen toward the direction of arrow 300 in FIG. 26(a), e.g., the R sub-field image 212 of the box image is presented to the observer's retinas in a certain moment, and the G sub-field image 214 of the box image is presented to the observer's retinas in the next moment and the B sub-field image 216 of the box image is presented to the observer's retinas in the next moment. Therefore, the three images of R, G and B are not synthesized so as to be completely coincident with each other on the observer's retinas, and the three images are synthesized with shifted from each other, since the observer's eye move across the display screen. As a result, the sub-field images of R, G and B are synthesized so as not to be coincident with the position of each edge of the box image. Therefore, the color breakup artifact such that the sub-field images of R, G and B are seen with separated colors is recognized. Such a phenomenon gives viewing stress or fatigue to the observer when the display unit is watched for a long time.
It is known that it is effective to increase the sub-field frequency in order to reduce the color breakup artifact. However, there is a limit of increasing the sub-field frequency due to the response time of a liquid crystal display or the like, and it is difficult to provide such a circuit, so that this is not preferred means.
On the other hand, there is proposed a method that an achromatic color signal (W signal) sub-field is added to the RGB sub-fields with a quadruple sub-field frequency (see Japanese Patent Laid-Open No. 8-101672). In this method, the minimum value of the RGB signals in each pixel per frame is displayed in a W sub-field, and chromatic color components which is differences between the value in the W sub-field and the original RGB signals are displayed in the RGB sub-fields. According to this method, most of signal components are displayed in the W sub-field in the case of an achromatic color components having high luminance, i.e., in the case of displaying a bright and whitish image, so that it is theoretically difficult to recognize color breakup. However, the aforementioned method can hardly obtain this effect in the case of displaying an image which comprises chromatic color components, e.g., in the case of displaying an image comprises many R and B signals and hardly contains G signals. For example, if the yellow (which will be hereinafter referred to as Y) components are dominant in an input picture signal, the color breakup between R and G is easily recognizable.